treewide: set shebang for bash scripts

These scripts previously failed when called with syscalls like
`execve` (used by, e.g., Python's `subprocess.run`) that use no default
interpreter for scripts without a shebang.

modules/lightning-loop.nix

@@ -73,7 +73,7 @@ let
       '';
     };
     cli = mkOption {
-      default = pkgs.writeScriptBin "loop" ''
+      default = pkgs.writers.writeBashBin "loop" ''
         ${cfg.package}/bin/loop \
         --rpcserver ${nbLib.addressWithPort cfg.rpcAddress cfg.rpcPort} \
         --macaroonpath '${cfg.dataDir}/${network}/loop.macaroon' \